Allow to specify a custom logger on Runner

Hello,

This commit allow to use a custom logger with a Goliath::Runner.

``` ruby
runner = Goliath::Runner.new(ARGV, nil)
runner.api = Custom.new
runner.app = Goliath::Rack::Builder.build(Custom, runner.api)
runner.logger = CustomLogger.new
runner.run
```

Cheers!